1. Field of the Invention
This invention relates to an air classification system for powdered materials and more particularly to an air classification system for the separation of ultra fine particles with a sharply defined topsize.
2. Prior Art
In the field of fine grinding and pulverization of dry industrial minerals and ores there is a need for precise classification of powders according to their size. An air classifier can be employed to perform separation according to particle size when the particles are generally too small to be separated by simple screening devices. The effectiveness of an air classifier may be measured by:
a) The ability to produce an ultra fine product (say with an equivalent spherical diameter of less than 10 microns) with a sharply defined topsize without the presence of small amounts of stray oversize particles.
b) The ability of the classifier to recover a reasonably high percentage of these fine particles that exist in the material to be classified.
A number of classification systems exist for the separation of powdered materials. (Two such devices are disclosed U.S. Pat. Nos. 4,257,890 and 4,296,864).
These and other such devices make use of rotating assemblies of blades or vanes which are used to impart centrifugal force on the particles to be classified when suspended in an air stream. Although such blades or vanes are an effective means of providing sharp particle classification, in all cases there is a need for a seal or barrier between the rotating and stationery parts of the classifier to protect the oversize particles from "leaking" into the fine product. The ability of such devices to produce ultra fine sizes in the fine classified product has been hereto limited by inefficient seal design which in turn leads to the use of relatively low air volumes resulting in poor fine product yields. Such seals or barriers regardless of their complexity or simplicity are never able to eliminate 100% of the oversize particles from entering into the fine classified product. An internal seal or barrier by its very nature is prone to high wear rates and is consistently in need of adjustment or maintenance.
The present invention creates a unique approach to separating the stationery and rotating parts of an air classifier which eliminates the need for internal seals or barriers and consequently allows the rotating element to improve the sharpness of classification and allows for maximized air flows resulting in higher fine product yields.